How fast your airplane flies? If you don’t have a speed radar gun then here is a practical way to calculate it with the airplane firmly secured on the ground.
1. With the engine properly tuned, measure the engine’s revs using a digital tachometer.
2. Take a note of the propeller’s pitch. As an example note 12 inches.
3. Measure the propeller’s revolutions with the airplane’s engine at full speed. As an example note 6.750 revs/min.
4. Multiply the above number of revolutions by the propeller’s pitch. i.e 6.750 x 12 = 81.000 (inches/min).
5. Multiply 81.000 (inches) x 60 (minutes) = 4.860.000 (inches/hour).
6. Divide by 2,54 (inch match to centimeter) 4.860.000/2,54= 1913 meters/min, which is 1,913 Km/min.
7. Multiply by 60 (min): 1,913 x 60= 114,78 Km/hour.
8. Multiply by the factor 0,95 (as note): 114,78 x 0,95 = 109,41 Km/hour.
The ground speed is multiplied by a factor of 0,95 which depends on the model’s frontal area and its configuration (low wing or upper wing) in order to calculate the model’s air speed.